Why the middle mouse button in Blender 2.80 changes the orthographic view to perspective?
It seems that you have Auto Perspective enabled, which causes the switch to perspective projection once you're changing the view from the top/front/left/right/bottom viewpoint. Open the preferences (Edit > Preferences) and disable Auto Perspective in the Navigation tab.



The defaults for Blender 2.80 where discussed in the design task T54943. Currently I haven't found any reasoning behind the changed default for Auto Perspective on developer.blender.org, bf-committers or devtalk.